(1) Field of the Invention
The invention generally relates to a television transmission system for transmitting television pictures in a digital format from an encoding station to a decoding station via some transmission medium.
More particularly the invention relates to a television transmission system in which generally picture coding and particularly transform coding are used to limit the bit rate.
Such a television transmission system may form part of a television broadcasting system, in which case the encoding station forms part of a broadcasting transmitter and each TV receiver is provided with a decoding station. In this case the transmission medium is the atmosphere.
Such a transmission system may also form part of a video recorder in which case the transmission medium is a videotape.
(2) Description of the Prior Art
In picture coding it is generally common practice to consider a picture as a matrix of E.sub.1 .times.E.sub.2 pixels, each with a given colour. As is known, a colour is obtained by a linear combination of three so-called component signals such as Y, U, V or R, G, B. The following applies to each of the component signals separately. The value which a given component signal has for a given pixel will be referred to as pixel value. When digitizing such a picture, a number is assigned to each pixel. This may indicate the pixel value itself, or, for example the difference between the pixel values of two contiguous pixels. In the first mentioned case a digital picture as in a canonical form, shortly called a canonical picture.
In a 625-line TV picture the visible part of the picture comprises 576 lines and each line comprises 720 pixels. If the considered pixel value for each pixel is represented by 8 bits, approximately 3.10.sup.6 bits are required for representing the canonical picture. With 25 such pictures per second this results in a bit rate of approximately 75.10.sup.6 bits/sec. In practice this is found to be unacceptably high. The object of the encoding station is to convert the canonical picture into a non-canonical picture which can be represented with a considerably smaller number of bits.
Different methods are known for the said conversion, for example the above-mentioned method in which a number is assigned to each pixel indicating the difference between the pixel values of two contiguous pixels. This method is known under the name of Differential Pulse Code Modulation, abbreviated DPCM. In another method, which is known under the name of transform coding, the canonical picture is subjected to a forward transform. To this end the picture to be transformed is more particularly partitioned into sub-pictures having a size of N.sub.p pixels. Each sub-picture is subsequently converted by means of a two-dimensional transform into N.sub.p coefficients. With the aid of such a picture transform a picture is thus converted into a matrix of E.sub.1 E.sub.2 coefficients. It is to be noted that the sub-pictures are usually square-shaped in practice.
A reduction of the number of bits which must be transmitted per picture is now realized by transmitting only a limited number of coefficients. This is only possible if the coefficients are uncorrelated. The transform chosen is closely related to the extent to which this object is achieved. In this connection the Karhunen-Loeve transform is found to be optimum (see for example Reference 1, pages 259-264). Nowadays Discrete Cosine Transform, abbreviated DCT is generally considered the best alternative (see Reference 1).
The total number of coefficients which is transmitted in this way for each picture forms a matrix and the number of coefficients in this matrix is referred to as the order of the picture transform.
In practice it is found that the number of coefficients qualifying for transmission may be greatly different from picture to picture. To prevent the allowed bit rate from being exceeded, the coefficients to be transmitted are quantized in a coarser manner as more coefficients are qualified for transmission. In this case the picture quality is, however, influenced detrimentally.